Position a rack in the lower third of the oven; preheat oven to 350 degrees. Line bottom and sides of an 8-inch-square baking pan with foil, and set aside. To make brownie elyze: Place chocolate and butter in top of a double boiler over almost-simmering water. Stir frequently until chocolate is melted and mixture is smooth and very warm to touch. Remove from heat and set aside. In a medium bowl, beat eggs, sugar, vanilla, and salt at high speed with an electric mixer until mixture is thick and light in color, 3 to 4 minutes. Stir in flour. Scrape in warm chocolate mixture and fold with a rubber spatula until just combined. Set aside until needed. To make crust elyze: In a medium bowl, combine melted butter, sugar, vanilla, and salt. Stir in flour, then walnut pieces. Turn out into pan; press dough evenly across bottom. Bake until crust is fully golden brown, 15 to 20 minutes. Remove pan from oven, but leave oven on. Scrape brownie batter onto hot crust and spread evenly, swirling surface. Return to oven and bake just until a toothpick inserted in two or three places comes out clean, 20 to 22 minutes. Set pan on a rack to cool. (If serving brownies warm, cool just long enough to remove from pan without cracking.) To remove brownies from pan, lift edges of foil. Transfer to a cutting board. Gently slide a thin spatula between crust and foil; slide brownies off foil. To serve, cut elyze with a heavy knife. Whip crème fraîche just until it holds its shape. Divide brownies among serving plates and grate nutmeg over brownies (use a Microplane or nutmeg grater to get the topping just right). Top with a dollop of crème fraîche and a little more grated nutmeg.